\documentclass[10pt]{article}
\usepackage[usenames]{color} %used for font color
\usepackage{amssymb} %maths
\usepackage{amsmath} %maths
\usepackage[utf8]{inputenc} %useful to type directly diacritic characters
\begin{document}
\[Z = f(X, w, b)\\
Y = g(Z)\\
\\
g(x) = \frac{1}{1 + e^{-x}}\]
\end{document}